Barding for a horse (size Large, non-humanoid) costs four times as much as the equivalent armor for a human and weighs twice as much.
Barding for a pony (size Medium, non-humanoid) costs twice as much as the equivalent armor for a human and weighs the same ammount as the equivalent armor.
Donning/removing barding takes five tiems as long as normal for a human to don/remove the equivalent armor and a mount wearing barding cannot carry any load other than the rider and saddle.
The general consensus on the boards the last time this subject came up was that the listed natural DR for most animals, and horses in particular, was stupidly high. (DR 6 for a Warhorse! :shock: ). I usually give medium animals a DR of 1-2, large 2-3 and huge 3-4.
Yes barding was used in the Hyborian age though mostly by the Hyborian knighthood themselves. I recall several Conan stories where there are Hyborian knights riding horses with barding.